1. Field of the Invention
This invention describes applications of monolithic 3D integration to semiconductor chips performing logic and memory functions.
2. Discussion of Background Art
Over the past 40 years, one has seen a dramatic increase in functionality and performance of Integrated Circuits (ICs). This may have largely been a result from the phenomenon of “scaling” i.e. component sizes within ICs have been reduced (“scaled”) with every successive generation of technology. There may be two main classes of components in Complementary Metal Oxide Semiconductor (CMOS) ICs, namely transistors and wires. With “scaling”, transistor performance and density typically improve and this may have contributed to the previously-mentioned increases in IC performance and functionality. However, wires (interconnects) that connect together transistors degrade in performance with “scaling”. The situation today is that wires dominate performance, functionality and power consumption of ICs.
3D stacking of semiconductor chips may be one avenue to tackle issues with wires. By arranging transistors in 3 dimensions instead of 2 dimensions (as was the case in the 1990s), one can place transistors in ICs closer to each other. This reduces wire lengths and keeps wiring delay low. However, there may be many barriers to practical implementation of 3D stacked chips. These include:                Constructing transistors in ICs typically require high temperatures (higher than about 700° C.) while wiring levels may be constructed at low temperatures (lower than about 400° C.). Copper or Aluminum wiring levels, in fact, can get damaged when exposed to temperatures higher than about 400° C. If one may like to arrange transistors in 3 dimensions along with wires, it may have the challenge described below. For example, a 2 layer stack of transistors and wires, i.e. Bottom Transistor Layer, above it Bottom Wiring Layer, above it Top Transistor Layer and above it Top Wiring Layer, may be arranged. When the Top Transistor Layer is constructed using Temperatures higher than about 700° C., it can damage the Bottom Wiring Layer.        As a result from the above mentioned problem with forming transistor layers above wiring layers at temperatures lower than about 400° C., the semiconductor industry has largely explored alternative architectures for 3D stacking. In these alternative architectures, Bottom Transistor Layers, Bottom Wiring Layers and Contacts to the Top Layer may be constructed on one silicon wafer. Top Transistor Layers, Top Wiring Layers and Contacts to the Bottom Layer may be constructed on another silicon wafer. These two wafers may be bonded to each other and contacts may be aligned, bonded and connected to each other as well. Unfortunately, the size of Contacts to the other Layer may be large and the number of these Contacts may be small. In fact, prototypes of 3D stacked chips today utilize as few as about 10,000 connections between two layers, compared to billions of connections within a layer. This low connectivity between layers may be because of two reasons: (i) Landing pad size may need to be relatively large as a result from alignment issues during wafer bonding. These may result from many reasons, including bowing of wafers to be bonded to each other, thermal expansion differences between the two wafers, and lithographic or placement misalignment. This misalignment between two wafers may limit the minimum contact landing pad area for electrical connection between two layers; (ii) The contact size may need to be relatively large. Forming contacts to another stacked wafer typically involves having a Through-Silicon Via (TSV) on a chip. Etching deep holes in silicon with small lateral dimensions and filling them with metal to form TSVs may not be easy. This places a restriction on lateral dimensions of TSVs, which in turn impacts TSV density and contact density to another stacked layer. Therefore, connectivity between two wafers may be limited.        
It may be highly desirable to circumvent these issues and build 3D stacked semiconductor chips with a high-density of connections between layers. To achieve this goal, it may be sufficient that one of three requirements must be met: (1) A technology to construct high-performance transistors with processing temperatures below about 400° C.; (2) A technology where standard transistors may be fabricated in a pattern, which allows for high density connectivity despite the misalignment between the two bonded wafers; and (3) A chip architecture where process temperature increase beyond about 400° C. for the transistors in the top layer does not degrade the characteristics or reliability of the bottom transistors and wiring appreciably. This patent application describes approaches to address (1), (2) and (3) in the detailed description section. In the rest of this section, background art that has previously tried to address (1), (2) and (3) may be described.
U.S. Pat. No. 7,052,941 from Sang-Yun Lee (“S-Y Lee”) describes methods to construct vertical transistors above wiring layers at less than about 400° C. In these single crystal Si transistors, current flow in the transistor's channel region is in the vertical direction. Unfortunately, however, almost all semiconductor devices in the market today (logic, DRAM, flash memory) utilize horizontal (or planar) transistors as a result from their many advantages, and it may be difficult to convince the industry to move to vertical transistor technology.
A paper from IBM at the Intl. Electron Devices Meeting in 2005 describes a method to construct transistors for the top stacked layer of a 2 chip 3D stack on a separate wafer. This paper is “Enabling SOI-Based Assembly Technology for Three-Dimensional (3D) Integrated Circuits (ICs),” IEDM Tech. Digest, p. 363 (2005) by A. W. Topol, D. C. La Tulipe, L. Shi, et al. (“Topol”). A process flow is utilized to transfer this top transistor layer atop the bottom wiring and transistor layers at temperatures less than about 400° C. Unfortunately, since transistors may be fully formed prior to bonding, this scheme suffers from misalignment issues. While Topol describes techniques to reduce misalignment errors in the above paper, the techniques of Topol still suffer from misalignment errors that limit contact dimensions between two chips in the stack to greater than about 130 nm.
The textbook “Integrated Interconnect Technologies for 3D Nanoelectronic Systems” by Bakir and Meindl (“Bakir”) describes a 3D stacked DRAM concept with horizontal (i.e. planar) transistors. Silicon for stacked transistors may be produced using selective epitaxy technology or laser recrystallization. Unfortunately, however, these technologies have higher defect density compared to standard single crystal silicon. This higher defect density degrades transistor performance.
In the NAND flash memory industry, several organizations have attempted to construct 3D stacked memory. These attempts predominantly use transistors constructed with poly-Si or selective epi technology as well as charge-trap concepts. References that describe these attempts to 3D stacked memory include “Integrated Interconnect Technologies for 3D Nanoelectronic Systems”, Artech House, 2009 by Bakir and Meindl (“Bakir”), “Bit Cost Scalable Technology with Punch and Plug Process for Ultra High Density Flash Memory”, Symp. VLSI Technology Tech. Dig. pp. 14-15, 2007 by H. Tanaka, M. Kido, K. Yahashi, et al. (“Tanaka”), “A Highly Scalable 8-Layer 3D Vertical-Gate (VG) TFT NAND Flash Using Junction-Free Buried Channel BE-SONOS Device,” Symposium on VLSI Technology, 2010 by W. Kim, S. Choi, et al. (“W. Kim”), “A Highly Scalable 8-Layer 3D Vertical-Gate (VG) TFT NAND Flash Using Junction-Free Buried Channel BE-SONOS Device,” Symposium on VLSI Technology, 2010 by Hang-Ting Lue, et al. (“Lue”) and “Sub-50 nm Dual-Gate Thin-Film Transistors for Monolithic 3-D Flash”, IEEE Trans. Elect. Dev., vol. 56, pp. 2703-2710, November 2009 by A. J. Walker (“Walker”). An architecture and technology that utilizes single crystal Silicon using epi growth is described in “A Stacked SONOS Technology, Up to 4 Levels and 6 nm Crystalline Nanowires, with Gate-All-Around or Independent Gates (ΦFlash), Suitable for Full 3D Integration”, International Electron Devices Meeting, 2009 by A. Hubert, et al (“Hubert”). However, the approach described by Hubert may have some challenges including the use of difficult-to-manufacture nanowire transistors, higher defect densities may result from the formation of Si and SiGe layers atop each other, high temperature processing for long times, and difficult manufacturing.
It is clear based on the background art mentioned above that invention of novel technologies for 3D stacked chips may be useful.